Surface wave components, so-called SAW components, are currently used in a variety of technical applications. Thus, SAW components operating according to the delay line principle are used as identification elements, so-called ID tags. However, SAW sensors are also used in order to detect or to monitor various measuring parameters. Measurements that may be carried out with such SAW sensors include, in particular, temperature measurements or also pressure measurements, but also (mechanical) stress measurements and force measurements. SAW components in such case have, in general, the particular advantage that they are comparatively robust and may also be used under adverse environmental conditions such as, for example, at high temperatures.
SAW components of this type are typically wirelessly remotely interrogated. For this purpose, two fundamentally different type of components and interrogation methods are frequently used:
So-called SAW resonators are excited at a frequency, typically, several times in succession at an excitation frequency varying within one frequency band, wherein these resonators respond at a sensor-typical resonance frequency, which—when the SAW resonators are used as sensor elements—displays a dependency on the measured value to be detected with the sensor, for example, a measuring temperature, in addition to a dependency on the sensor geometry. The response signal received in return by the SAW component is evaluated with respect to its frequency position, and on the basis of this position, the measured value is deduced.
In practice, the various response signals to the excitation signals varied in the frequency band are observed in the process, wherein an actual measurement result is obtained, for example, averaged, from the various response signals.
A second type of SAW components operates according to the delay line principle, they are also referred to as “Delay line SAWs”. In the case of these components, the SAW component, for example, a SAW sensor, emits a response at the same frequency, as that of the excitation signal, the information about the data transmitted back by the SAW component is contained in the time delay of the response signal relative to the excitation, the so-called “delay”. This delay may, for example, simply express an identification, is modified when such SAW components are used as SAW sensors depending on the state of the observed parameters, for example, depending on the temperature to be detected by such a SAW sensor. The aim is to determine and evaluate this delay accordingly.
SAW components that operate according to the delay line principle are currently read out, inter alia, using a method in which a high frequency signal, for example, a signal having a frequency in the range of 2.4 to 2.5 GHz, in particular, having a frequency from the range of 2.4 to 2.4835 GHz, is emitted as a signal pulse for exciting the SAW component, a response signal pulse of the SAW component is received, this response signal of the SAW component is mixed in a mixer with a high frequency signal originating from a local oscillator (LO), as it is also supplied for the signal pulse of the excitation, and the output signal of the mixer, which is actually a direct current signal, is evaluated to determine a piece of data to be read out, whether this is a simple identification or also a piece of sensor data, for example, a temperature value. Thus, a homodyne detection is accordingly used in this case. This approach is described, for example, in the article “Readout Unit for Wireless SAW Sensors and ID-Tags” by the authors Andreas Stelzer, Stefan Schuster, Stefan Scheiblhofer in “Proc. 2nd Int. Symp. Acoust. Wave Dev. for Future Mobile Comm. Syst.”, (Chiba, Japan), March 2004, pages 37-44. Explanations on the reading out of SAW sensors operating according to the delay line principle are also found in DE 602 03 805 T2 in the general introduction and in the description of the prior art described therein, as well as in U.S. Pat. No. 8,240,911 B1, in particular, in FIG. 4 and in the related description.
The problem with this approach is that during this mixing of the response signal of the component and of the excitation signal, signal parts which overlay the response signal, for example, parasitic couplings of the transmission frequency into the receive path and, in addition, al/f noise signal also arriving in the receive path, likewise in fact mixed down to a 0 frequency, are thus transferred into a DC component, so that these signal parts also form part of the resultant direct current signal, which is intended to represent the measurement result. However, this parasitic contribution can no longer be eliminated from the direct current signal, such that it results in a significant measurement error. In such case, errors of several percent occur, in extreme cases up to 30%. In practice, therefore, considerable effort is made to prevent parasitic couplings of the transmission signal into the receive path on the one hand, and to suppress 1/f noise to the extent possible on the other hand. These efforts result in complex structured and, therefore, expensive reading devices, but in practice are then also not always able to prevent errors in the determination of the transmitted data.
These fundamental considerations apply, in principle, not only to SAW components, but to any other possible passive type of components or elements, which operates with a corresponding response according to the delay line principle.